Conventionally, clay is blended into rubber compositions for a tire for the purpose of securing a weighting effect (cost reduction) or molding stability. However, rubber compositions containing clay instead of carbon black have had problems in that the rubber hardness and strength are diminished in comparison to rubber compositions containing carbon black.
Japanese Unexamined Patent Application Publication No. 2002-80638A proposes a rubber composition for a tire tread containing a clay having an average particle size of at most 10 μm and carbon black having a nitrogen adsorption specific surface area from 70 to 300 m2/g.
However, the processability may be diminished in a rubber composition containing a clay having an average particle size of at most 10 μm, and a rubber composition for a tire which simultaneously achieves the rubber hardness, strength, and processability has not yet been established.